Mr Sessions said on Thursday: "I would cite you to the Apostle Paul and his clear and wise command in Romans 13, to obey the laws of the government because God has ordained the government for his purposes.

"Our policy that can result in short-term separation of families is not unusual or unjustified."

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Media captionLorry full of migrants found in Texas

White House spokeswoman Sarah Sanders would not comment directly on Mr Sessions' remarks, but added "it's very biblical to enforce the law".

In the House, top Democrat Nancy Pelosi attacked the separations as a "barbaric" policy that "has to stop."

The Trump administration policy is supported by some Republicans, but others have expressed misgivings.

Speaking to reporters on Thursday, House Speaker Paul Ryan, Congress' highest-ranking Republican, was asked if he was comfortable with the tactics.

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Media captionMigrant boys detained in a former Walmart in Texas

"No I am not," he responded. "We believe it should be addressed in immigration legislation.

"We don't want kids to be separated from their parents."

The policy has also provoked disquiet among the conservative evangelical community.

Franklin Graham, son of late reverend Billy Graham and a staunch Trump supporter, said the separation policy was "disgraceful".

"It's terrible to see families ripped apart and I don't support that one bit," Graham told the Christian Broadcasting Network on Tuesday.

The United States Conference of Catholic Bishops also condemned the policy on Wednesday.

"Our government has the discretion in our laws to ensure that young children are not separated from their parents and exposed to irreparable harm and trauma.

"While protecting our borders is important, we can and must do better as a government, and as a society, to find other ways to ensure that safety."

On Twitter, many criticised Mr Sessions' citing of the verse given its historical use to justify slavery.